This invention relates to signal processing apparatuses and methods for use with a plurality of cable lines, e.g. cable lines including one or more twisted pair cables. In particular, this invention relates to apparatuses and methods for analysing one or more characteristics of a test signal coupled out from one of a plurality of cable lines. In some examples, the invention relates to a network interconnection identification apparatus for identifying interconnections between ports in a network.
Cables which include a plurality of twisted pairs, referred to as “twisted pair cables” herein, are well known. Such cables are commonly used for telecommunications purposes, e.g. computer networking and telephone systems. In the field of telecommunications, twisted pair cables are usually provided without shielding, as unshielded twisted pair (UTP) cables. However, shielded twisted pair (STP) cables are also known.
In this context, a “twisted pair” is a pair of conductors, usually a forward conductor and a return conductor of a single circuit, which have been twisted together. The conductors are usually twisted together for the purposes of cancelling out electromagnetic interference from external sources and to minimise crosstalk between neighbouring twisted pairs within a cable comprising a plurality of twisted pairs. In this way, each twisted pair provides a reliable respective communication channel for a signal, usually a differential voltage signal, to be conveyed within the twisted pair. Common forms of unshielded twisted pair (UTP) cables are category 5 and category 6 UTP cables which include eight conductors twisted together in pairs to form four twisted pairs.
The design and construction of twisted pair cables is carefully controlled by manufacturers to reduce noise due to electromagnetic interference and to reduce crosstalk between the twisted pairs within the cables. To this end, each twisted pair in a twisted pair cable normally has a different twist rate (i.e. number of twists per unit length along the cable) from that of the other twisted pairs in the twisted pair cable. It is also usual for the twisted pairs to be twisted around each other within the cable. Fillets or spacers may be used to separate physically the twisted pairs.
Networks including ports interconnected by a plurality of cables, such as local area networks (LANs), are also well known. LANs are typically used to enable equipment such as computers, telephones, printers and the like to communicate with each other and with remote locations via an external service provider. LANs typically utilise twisted pair network cables, usually in the form of UTP cables.
The cables used in LANs are typically connected to dedicated service ports throughout one or more buildings. The cables from the dedicated service ports can extend through the walls, floor and/or ceilings of the building to a communications hub, typically a communications room containing a number of network cabinets. The cables from wall and floor sockets within the building and from an external service provider are also usually terminated within the communications room.
A “patch system” may be used to interconnect various ports of the LAN within the network cabinets. In a patch system, all cable lines in the LAN can be terminated within the network cabinets in an organized manner. The terminations of the cable lines in the network are provided by the structure of the network cabinets, which are typically organised in a rack system. The racks contain “patch panels”, which themselves utilise sets of ports, typically RJ-45 type connector ports, at which the cable lines terminate.
Each of the ports in each patch panel is hard wired to one of the cable lines in the LAN. Accordingly, each cable line is terminated on a patch panel in an organized manner. In small patch systems, all cable lines in the LAN may terminate on the patch panels of the same rack. In larger patch systems, multiple racks are used, wherein different cable lines terminate on different racks.
Interconnections between the various ports in the LAN are typically made using “patch cables”, which are usually UTP cables including four twisted pairs. Each end of a patch cable is terminated by a connector, such as an RJ-45 type connector for inserting into an RJ-45 type connector port. One end of each patch cable is connected to the port of a first cable line and the opposite end of the patch cable is connected to the port of a second cable line. By selectively connecting the various cable lines using the patch cables, a desired combination of network interconnections can be achieved.
FIG. 12 shows a typical patch system organised into a server row 92, a cross-connect row 93 and a network row 94, which include patch panels 96a, 96b, 96c, 96d. Patch cables 10a, 10b, 10c, 10d are used to interconnect two ports through the patch system.
In many businesses, employees of a company are assigned their own computer network access number so that the employee can interface with the company's IT infrastructure. When an employee changes office locations, it is not desirable to provide that employee with newly addressed port in the network. Rather, to preserve consistency in communications, it is preferred that the exchanges of the ports in the employee's old office be transferred to the telecommunications ports in the employee's new location. This type of move is relatively frequent. Similarly, when new employees arrive and existing employees depart, it is usually necessary for the patch cables in the network cabinet(s) to be rearranged so that each employee's exchanges can be received in the correct location.
As the location of employees change, the patch cables in a typical cabinet are often manually entered in a computer based log. This is burdensome. Further, technicians often neglect to update the log each and every time a change is made. Accordingly, the log is often less than 100% accurate and a technician has no way of reading where each of the patch cables begins and ends. Accordingly, each time a technician needs to change a patch cable, that technician manually traces that patch cable between an internal line and an external line. To perform a manual trace, the technician locates one end of a patch cable. The technician then manually follows the patch cable until he/she finds the opposite end of that patch cable. Once the two ends of the patch cable are located, the patch cable can be positively identified.
It takes a significant amount of time for a technician to manually trace a particular patch cable, especially in large patch systems. Furthermore, manual tracing is not completely accurate and a technician may accidently go from one patch cable to another during a manual trace. Such errors result in misconnected patch cables which must be later identified and corrected.
Attempts have been made in the prior art to provide an apparatus which can automatically trace the common ends of each patch cable within local area networks, thereby reducing the labour and inaccuracy of manual tracing procedures.
For example, U.S. Pat. No. 5,483,467 describes a patching panel scanner for automatically providing an indication of the connection pattern of the data ports within a LAN, so as to avoid the manual task of identifying and collecting cable connection information. In one embodiment, which is intended for use with shielded twisted pair cables, the scanner uses inductive couplers which are associated with the data ports. The inductive coupler is disclosed as being operative to impose a signal on the shielding of shielded network cables in order to provide an indication of the connection pattern produced by connection of the cables to a plurality of ports.
In another embodiment of U.S. Pat. No. 5,483,467, the scanner is coupled to each data port by “dry contact” with a dedicated conductor in a patch cable. This is difficult to implement in practice, because most network cables have to meet a particular pre-determined standard in the industry, such as the RJ-45 type standard, in which there is no free conductor which could be used for determining interconnectivity.
U.S. Pat. No. 6,222,908 discloses a patch cable identification and tracing system in which the connectors of each patch cable contain a unique identifier which can be identified by a sensor in the connector ports of a telecommunications closet. By reading the unique identifier on the connectors of each patch cable, the system can keep track of which patch chords are being added to and removed from the system. Although this system avoids the use of dedicated conductors in the patch cable, it is difficult to implement because it requires use of non-standard patch cables, i.e. patch cables with connectors containing unique identifiers.
International Patent Application Publication Number WO00/60475 discloses a system for monitoring connection patterns of data ports. This system uses a dedicated conductor which is attached to the external surface of a network cable in order to monitor the connection pattern of data ports. Although this allows the system to be used with standard network cables, it still requires the attaching of dedicated conductors to the external surfaces of network cables and adapter jackets which are placed over the standard network cable.
U.S. Pat. No. 6,285,293 discloses another system and method for addressing and tracing patch cables in a dedicated telecommunications system. The system includes a plurality of tracing interface modules that attach to patch panels in a telecommunications closet. On the patch panels, are located a plurality of connector ports that receive the terminated ends of patch cables. The tracing interface modules mount to the patch panels and have a sensor to each connector port which detects whenever a patch cable is connected to the connector port. A computer controller is connected to the sensors and monitors and logs all changes to the patch cable interconnections in an automated fashion. However, this system cannot be retrofitted to an existing network and relies on the operator to work in a particular order if the patch cable connections are to be accurately monitored.
International Patent Application Publication Number WO2005/109015, which relates to the field of cable state testing, discloses a method of determining the state of a cable comprising at least one electrical conductor and applying a generated test signal to at least one conductor of the cable by a non-electrical coupling transmitter. The reflected signal is then picked up and compared with expected state signal values for the cable, so that the state of the cable can be determined. The present inventors have found that signals coupled to a twisted pair cable by the methods described in WO2005/109015 have a tendency to leak out from the twisted pair cable, especially when other twisted pair cables are nearby.
UK patent application number GB0905361.2, U.S. patent application Ser. No. 11/597,575 and International patent application number PCT/GB2010/000594, also by the present inventors, and the content of which is herewith incorporated in its entirety, describe apparatuses and methods for coupling a signal to and from a twisted pair cable by non-contact coupling with twisted pairs in the twisted pair cable, such that the signal propagates along the cable between at least two of the twisted pairs. These methods and apparatuses related to a discovery by the present inventors that a twisted pair cable provides communication channels (referred to as “pair-to-pair channels”) between the twisted pairs within the twisted pair cable, these communication channels being additional to the respective communication channel provided within each twisted pair in the cable. In other words, a signal propagating between the twisted pairs could be transmitted on a standard UTP cable (i.e. with no specially adapted UTP cable needed) without interfering with signals propagating within individual twisted pairs of the standard UTP cable.
“Crosstalk” is a common problem within the field of networks. “Crosstalk” may be viewed as undesired signal coupling from one signal channel to another. For networks including a plurality of twisted pair cables, crosstalk may occur between the twisted pairs within individual twisted pair cables, but most commonly occurs between twisted pairs in different twisted pair cables, particularly when the twisted pairs have a similar twist rate. In industry, crosstalk between different twisted pair cables is sometimes referred to as “alien” crosstalk and is illustrated by FIG. 1b, which is described in more detail below.
The presence of crosstalk between twisted pair cables can cause serious difficulties for the operation of a network as it can cause signals to be routed via undesired paths between ports in the network. To address these difficulties, cables (e.g. category 5 and category 6 UTP cables), connectors and associated components are normally manufactured to ensure that levels of crosstalk are maintained within prescribed standards. In addition, cables must often be installed and connected according to guidelines in order to ensure that the network performs to the prescribed standards.